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A few years ago, dog trainers and behaviorists renewed their love affair with tail-wagging, constantly checking to see whether dogs were wagging their tails higher to the right or to the left. Our awkward attempts at positioning ourselves to observe this behavior were surely entertaining to others. Why were we so eager for the information conveyed by these asymmetrical tail wags? Because they indicate dogs’ differential use of the left and right hemispheres of their brains and are, therefore, a window into their emotions.

The study of asymmetrical tail wagging that prompted our collective interest (Quaranta et al. 2007) found that differences depended on what inspired the wags in the first place. Dogs wagged higher to the right when greeting their guardians. The same right-side bias was seen in response to unfamiliar people, although the wags were lower overall. In response to cats, there was little wagging, but it was still higher to the right. In the tests, the only stimulus to which dogs’ wags had a left-side bias was an unfamiliar, confident dog.

Left or Right?
Asymmetrical tail wags reflect the way the two sides of the brain process information and affect the body. The right hemisphere controls the left side of the body and the left hemisphere controls the right side. When dogs wag their tails to the right, they are engaging the muscles on the right side of their body more actively than those on their left; this demonstrates greater involvement of the left hemisphere of the brain.

The left hemisphere is activated when the brain is processing positive experiences associated with emotions such as happiness, affection and excitement, as well as anything familiar. The right hemisphere takes precedence when processing sadness, fear, other negative emotions and novel things.

This link between emotions and sides of the brain came to light in studies of humans. Ahern and Schwartz (1979) found that people who were asked questions that elicited either positive or negative emotions responded in accordance with this principle. They looked to their right (showing left brain hemisphere involvement) in response to questions that elicited positive emotions, but looked to their left (showing right brain hemisphere involvement) in response to questions that evoked negative emotions.

Individuals—canine or human—who favor the left paw or hand more often use the right hemisphere of their brain, while right-pawed and right-handed individuals have a more active left-brain hemisphere. Studies have shown differences between right-pawed and left-pawed dogs. They have also revealed that dogs who are ambilateral—who don’t have a paw preference—are different in predictable ways from dogs who strongly prefer one paw over the other.

Lateralization research, an active area of study, informs our understanding of emotions and behavior. Though dogs and people are common study subjects, similar patterns have been found in fish, amphibians, reptiles, birds, and primates and other mammals.
We now know that the significance of brain lateralization, handedness and paw preference extends far beyond matters of scissors and can-openers (people) and learning to shake (dogs). There are strong links between paw preference, the strength of that preference, and the behavior and emotional life of dogs.

Determining Paw Preference
In humans, we identify hand preference based on which hand a person uses to eat, write and so forth or by seeing who keeps their arms tucked in tight when eating at a small round table. (It’s the lefties, because they are used to colliding with the righties next to them if they don’t act to prevent it.) In dogs, most determinations are based on the “Kong test,” in which dogs are observed extracting food from a Kong. Every time the dog uses a paw to stabilize the Kong, the observer records which paw was used. If the dog uses both paws simultaneously, that is also recorded. From these data, researchers determine a dog’s paw preference as well as the strength of that preference. There are approximately equal numbers of left-pawed, right-pawed and ambilateral dogs, which is different than the preponderance of righties in humans.